Objective: RNA tumor viruses represent a group of agents known to be associated with spontaneous neoplasias in several animal species and possibly in man. The long-term goal of the proposed project is to clarify the mechanisms by which these viruses cause malignant transformation. Approach: The research approach to this goal is a genetic analysis of avian DNA tumor viruses using conditional (temperature-sensitive) and non-conditional mutants of different strains of RSV. This research will involve: (1) The isolation of temperature-sensitive (TS) mutants from different strains of RSV, (2) Physiological characterization of the mutants, and (3) Complementation and recombination studies with these mutants.